diff --git a/cellxgene_gateway/items/s3/s3item_source.py b/cellxgene_gateway/items/s3/s3item_source.py
index bcf7d39..39abe03 100644
--- a/cellxgene_gateway/items/s3/s3item_source.py
+++ b/cellxgene_gateway/items/s3/s3item_source.py
@@ -29,6 +29,8 @@ class S3ItemSource(ItemSource):
     ):
         self._name = name
         self.s3 = s3fs.S3FileSystem()
+        if bucket.startswith('s3://'):
+            raise Exception(f"Bucket name should not include s3:// prefix, got {bucket}")
         self.bucket = bucket
         self.h5ad_suffix = h5ad_suffix
         self.annotation_dir_suffix = annotation_dir_suffix
@@ -67,7 +69,7 @@ class S3ItemSource(ItemSource):
             raise Exception(f"S3 url '{url}' does not exist.")
 
         s3key_map = dict(
-            (filepath[len(self.bucket) :], "s3://" + filepath)
+            (filepath[len(self.bucket) :].lstrip('/'), "s3://" + filepath)
             for filepath in sorted(self.s3.ls(url))
         )